Resistance spot welding device

ABSTRACT

A resistance spot welding device for welding at least two overlapping steel sheets held between a pair of welding electrodes is provided. The resistance spot welding device includes the pair of electrodes, an electrode force gauge that measures an electrode force, and a controller that controls an electric current supply to the electrodes according to the electrode force measured by the electrode force gauge. The controller controls the electric current such that the electrode force F measured by the electrode force gauge after the start of the electric current supply is adjusted to a prescribed value.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2016/001174, filed Mar. 3, 2016, which claims priority to Japanese Patent Application No. 2015-043053, filed Mar. 5, 2015, and Japanese Patent Application No. 2015-071650, filed Mar. 31, 2015, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a resistance spot welding device for resistance spot welding, which is one type of lap resistance welding.

BACKGROUND OF THE INVENTION

To achieve an improvement in reliability of vehicle bodies and a reduction in weight of the vehicle bodies for the purpose of an improvement in fuel consumption, strengthening of steel sheets has been pursued in recent years. The use of high-strength steel sheets allows vehicle bodies to have stiffness comparable to that obtained using conventional steel even when the steel sheets are reduced in thickness and weight. However, some problems have been pointed out. One of them is a reduction in the strength of welded portions.

As shown in FIG. 1, in resistance spot welding, a sheet set 3 including at least two overlapping steel sheets (in this case, two steel sheets including a lower steel sheet 1 and an upper steel sheet 2) is held between a vertical pair of electrodes (a lower electrode 4 and an upper electrode 5), and an electric current is applied under pressure to melt a contact region between the steel sheets 1 and 2. A nugget 9 of the required size is thereby formed, and a weld joint is obtained.

The quality of the joint obtained in the manner described above is evaluated by, for example, the diameter of the nugget, tensile shear strength (the strength when a tensile test is performed in a shear direction of the joint), cross-tension strength (the strength when a tensile test is performed in a separation direction of the joint), or fatigue strength. In particular, as the strength of a steel sheet increases, the amount of C in the steel sheet tends to increase. It is known that, in a high-strength steel sheet containing a large amount of C, its cross-tension strength is low.

In terms of the welding method, to ensure the cross-tension strength when the high-strength steel sheet is used, it is contemplated to increase the diameter of the nugget. Generally, to increase the size of the nugget, an electric current must be increased. In this case, the possibility of the occurrence of expulsion becomes high. If expulsion occurs, the nugget is rather reduced in size, and this causes a reduction in the strength of the joint.

In particular, the surfaces of steel sheets for automobiles are subjected to galvanization treatment with zinc as a main component for the purpose of rust prevention. It is known that, when steel sheets having such galvanized layers are used for automobiles and subjected to resistance spot welding to assemble the automobiles, expulsion is likely to occur, and it is therefore difficult to ensure large nuggets.

A conventional technique disclosed in Patent Literature 1 is a method for forming nuggets in three stacked steel sheets. In this method, after a first welding step is performed, second and subsequent welding steps are performed such that the supply of electric current and the suspension of the supply are repeated in a pulsated manner. This allows nuggets with sufficient diameters to be formed even when the sheet set including three sheets is composed of a thin sheet, a thick sheet, and a thick sheet.

It is stated in Patent Literature 2 that, when steel sheets having on their surfaces alloyed aluminum coating layers containing Fe at an atomic percentage of from 50% to 80% inclusive are welded, a nugget can be formed stably by specifying, according to the thickness of the sheets, the period of time during which the current is held constant after upslope energization.

It is stated in Patent Literature 3 that, with zinc or zinc alloy coated steel sheets, a nugget having a certain size can be ensured by limiting the ratio of the period of preliminary energization to the period of the formation of the nugget.

It is stated in Patent Literature 4 that, with zinc or zinc alloy coated steel sheets, a nugget having a certain size can be ensured by, after preliminary energization, repeating cooling and energization using a current value higher than the current value for the preliminary energization.

PATENT LITERATURE

PTL 1: Japanese Patent No. 4728926

PTL 2: Japanese Unexamined Patent Application Publication No. 2011-167742

PTL 3: Japanese Patent No. 3849539

PTL 4: Japanese Patent No. 3922263

SUMMARY OF THE INVENTION

However, the methods described in Patent Literatures 1 to 4 still have a problem in that it is difficult to ensure a nugget diameter stably. In particular, at a welding site for actual automobile assembly, unintended work disturbances such as a sheet gap are present and affect the formation of a nugget. With the methods described in Patent Literatures 1 to 4, it is further difficult to ensure a nugget diameter stably when work disturbances are present.

In certain embodiments of the present invention a resistance spot welding device that can form a large nugget while the occurrence of expulsion is prevented, whereby the diameter of the nugget can be ensured stably even when work disturbances such as a sheet gap are present, is provided.

Resistance spot welding of a sheet set including high-strength steel sheets was repeatedly studied. In the studies, the electrode force applied between the electrodes during welding was measured in real time. Specifically, a pressure mechanism for applying pressure to the upper electrode was used to measure the value of the load applied between the electrodes during welding in real time (the load value is referred to as a measured electrode force or simply as an electrode force). The electrode force applied through the upper electrode can be set to a designated value (referred to as a designated electrode force). However, the measurement results showed that the designated electrode force differed from the electrode force applied between the electrodes during welding.

It was also determined that there is a close relation between the occurrence of expulsion and the electrode force. Specifically, in the case where the electrode force increased rapidly in an initial stage of the electric current supply, expulsion occurred when the electrode force exceeded a certain value. Then, after the electric current supply was suspended and the electrode force was reduced, the electric current supply was resumed. In this case, no expulsion occurred even when the electrode force was higher than that in the first electric current supply.

Examples of the results obtained by are shown FIGS. 2 and 3. FIG. 2 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while the electric current supply and the suspension of the supply were repeated. FIG. 3 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while a constant electric current was supplied. In these experiments, a pressure was applied for about 10 cycles (200 ms) before the electric current was supplied to thereby obtain a stable state, and then the electric current supply was started. The average electrode force in the first cycle (20 ms) after the start of the electric current supply is referred to as the initial electrode force.

As shown in FIG. 2, no expulsion occurred when the electric current supply and the suspension of the supply were repeated, and the diameter of the nugget obtained was large. However, as shown in FIG. 3, when a constant electric current was supplied continuously, expulsion occurred, and the diameter of the nugget was small.

Without being bound by theory, this mechanism may be as follows. In the initial stage of the electric current supply, a nugget is formed rapidly and expands, and this causes the measured electrode force to increase. Expulsion may occur when the pressurized state around the nugget is insufficient. Specifically, when the measured electrode force becomes equal to or larger than a prescribed value during the electric current supply, the relative pressure around the nugget decreases, and this may cause expulsion to occur. When the increase in the electrode force is small, which indicates the thermal expansion due to the formation of the nugget is small, and this results in an insufficient nugget diameter.

When the electric current supply is suspended, the nugget solidifies and contracts, and the measured electrode force decreases. At the same time, heat is transferred to the surroundings of the nugget, and the temperature of the portion around the nugget increases. This portion is thereby softened, and the pressurized state by the electrodes is ensured, so that the occurrence of expulsion can be prevented. However, as the period of time after the start of the suspension of the electric current supply increases, cooling proceeds, and it becomes difficult to form a nugget during the subsequent supply of the electric current.

In view of the above, whether a nugget with a large diameter can be formed without the occurrence of expulsion by utilizing the above phenomenon while the electrode force is controlled was studied. As a result, it was determined that in an initial stage of the supply of electric current, the electric current supply and the suspension of the supply are repeated, and the electrode force is controlled appropriately during the repetitions. This allows the final diameter of the nugget to be increased while the occurrence of expulsion is prevented.

Embodiments according to the present invention are as follows.

[1] A resistance spot welding device for welding a sheet set including at least two overlapping steel sheets by holding the sheet set between a pair of welding electrodes and supplying an electric current to the sheet set under pressure, the resistance spot welding device comprising:

a pair of electrodes;

an electrode force gauge that measures an electrode force F applied by the electrodes; and

a controller that controls an electric current supply to the electrodes according to the electrode force F measured by the electrode force gauge,

wherein the controller controls the electric current supply such that, when the electrode force F measured by the electrode force gauge after the electric current supply is started changes from an initial electrode force Fi to an electrode force F_(h) ⁽¹⁾ represented by formula (1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive, a suspension of the electric current supply for from 20 ms to 60 ms inclusive is started, and then,

the electric current supply is resumed when the electrode force F reaches an electrode force F_(c) ⁽¹⁾ represented by formula (2):

1.03×Fi≦F _(h) ⁽¹⁾≦1.15×Fi,   (1)

1.01×Fi≦F _(c) ⁽¹⁾≦0.99×F _(h) ⁽¹⁾.   (2)

[2] The resistance spot welding device according to [1], wherein the controller further controls the electric current supply such that,

after the.suspension of the electric current supply, the electric current supply for from 20 ms to 80 ms inclusive and the suspension of the electric current supply for from 20 ms to 60 ms inclusive are repeated at least once, and that,

when the electrode force F during an Nth electric current supply changes from an electrode force F_(c) ^((N−1)) immediately after an (N−1)th suspension of the electric current supply to an electrode force F_(h) ^((N)) represented by formula (3), the suspension of the electric current supply is started, and then,

the electric current supply is resumed when the electrode force F reaches an electrode force F_(c) ^((N)) represented by formula (4):

1.04×F _(c) ^((N−1)) ≦F _(h) ^((N))≦1.15×F _(c) ^((N−1)),   (3)

F _(c) ^((N−1)) ≦F _(c) ^((N))≦0.99×F _(h) ^((N)),   (4)

where N is a natural number of 2 or more.

[3] The resistance spot welding device according to [1] or [2], wherein the controller controls the electric current supply such that the period of a last N+1th repetition of the electric current supply is from 100 ms to 300 ms inclusive.

[4] The resistance spot welding device according to any of [1] to [3], further comprising a gun arm to which the pair of electrodes is mounted,

wherein the electrode force gauge is a strain gauge that measures strain of the gun arm.

According to embodiments of the present invention, when a sheet set including at least two overlapping steel sheets is subjected to resistance spot welding, a large nugget can be formed while the occurrence of expulsion is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing the outline of resistance spot welding.

FIG. 2 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while the supply of electric current and the suspension of the supply were repeated.

FIG. 3 is a graph showing changes in electrode force with respect to the initial electrode force when the resistance spot welding was performed while a constant electric current was supplied.

FIG. 4 is an illustration showing the structure of a resistance spot welding device according to an embodiment of the present invention.

FIG. 5 is an illustration showing an example of the structure of the resistance spot welding device according to the embodiment of the present invention, an electrode force gauge being composed of a strain gauge.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will next be described with reference to the accompanying drawings.

FIG. 4 is an illustration showing the structure of a resistance spot welding device according to the embodiment of the present invention. This device is a device for resistance spot welding in which a sheet set 3 including at least two overlapping steel sheets (1 and 2) is held between a pair of electrodes and welded by applying an electric current under pressure. This device includes a pair of electrodes (a lower electrode 4 and an upper electrode 5), an electrode force gauge 6, and a controller 7.

The electrode force gauge 6 measures an electrode force F applied by the electrodes 4 and 5 (hereinafter referred to simply as the electrode force F) and outputs the measured electrode force F to the controller 7. For example, the electrode force gauge 6 may be a strain gauge 61 that measures the strain of a gun arm 8 to which the lower electrode 4 and the upper electrode 5 are mounted, as shown in FIG. 5. When the electrode force F is applied, a strain approximately proportional to the electrode force F is generated in the gun arm 8. This strain is measured by the strain gauge 61 and converted to the electrode force F, and the electrode force F can thereby be determined.

The electrode force gauge 6 used may be, in addition to the strain gauge 61, a load cell capable of measuring the electrode force F, and the strain gauge 61 and the load cell may be used in combination.

The controller 7 controls the electrode force F according to the electrode force F measured by the electrode force gauge 6 as follows. Specifically, when the electrode force F after an electric current supply is started changes from an initial electrode force Fi to an electrode force F_(h) ⁽¹⁾ represented by formula (1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive, the controller 7 starts a suspension of the electric current supply for from 20 ms to 60 ms inclusive.

1.03×Fi≦F _(h) ⁽¹⁾≦1.15×Fi   (1)

If F_(h) ⁽¹⁾ is less than 1.03×F_(i), a region in the vicinity of the nugget is not pressurized sufficiently, and the probability of the occurrence of expulsion becomes high. If F_(h) ⁽¹⁾ is larger than 1.15×F_(i), the growth of the nugget is inhibited.

Then the controller 7 controls the electric current supply such that the electric current supply is resumed when the electrode force F reaches an electrode force F_(c) ⁽¹⁾ represented by formula (2).

1.01×Fi≦F _(c) ⁽¹⁾≦0.99×F _(h) ⁽¹⁾   (2)

If F_(c) ⁽¹⁾ is less than 1.01×Fi, cooling proceeds, and therefore the effects of the subsequent heating become small. If F_(c) ⁽¹⁾ is larger than 0.99×F_(h) ⁽¹⁾, the temperature of the nugget is high, and therefore the possibility of the occurrence of expulsion when the electric current supply is resumed increases.

Before the electric current is supplied, a pressure is applied for about 10 cycles (200 ms) to obtain a stable state, and then the electric current supply is started. The initial electrode force Fi used is the average electrode force in the first cycle (20 ms) after the start of the supply. The electrode force immediately after the start of the electric current supply is approximately the same as a set electrode force (designated electrode force) of a servo gun, and therefore the designated electrode force may be used as the initial electrode force Fi. Alternatively, the average electrode force in a period from 0 ms to 20 ms after the start of the electric current supply may be used as the initial electrode force Fi.

Then the controller 7 controls the electric current supply such that, after the suspension of the electric current supply, the electric current supply for from 20 ms to 80 ms inclusive and the suspension of the electric current supply for from 20 ms to 60 ms inclusive are repeated at least once.

Then, when the electrode force F during the Nth electric current supply changes from an electrode force F_(c) ^((N−1)) immediately after the (N−1)th suspension of the electric current supply to an electrode force F_(h) ^((N)) represented by formula (3), the controller 7 starts the suspension of the electric current supply.

1.04×F _(c) ^((N−1)) ≦F _(h) ^((N))≦1.15×F _(c) ^((N−1))   (3)

If F_(h) ^((N)) is less than 1.04×F_(c) ^((N−1)), the probability of the occurrence of expulsion becomes high. If F_(h) ^((N)) is larger than 1.15×F_(c) ^((N−1)), the growth of the nugget is inhibited.

Then the controller 7 controls the electric current supply such that, when the electrode force F reaches an electrode force F_(c) ^((N)) represented by formula (4), the electric current supply is resumed. N is a natural number of 2 or more.

F _(c) ^((N−1)) ≦F _(c) ^((N))≦0.99×F _(h) ^((N))   (4)

If F_(c) ^((N)) is less than F_(c) ^((N−1)), cooling proceeds, and therefore the effects of the subsequent heating become small. If F_(c) ^((N)) is larger than 0.99×F_(h) ^((N)), the possibility of the occurrence of expulsion when the electric current supply is resumed increases.

In embodiments of the present invention, the period of the last repetition of the electric current supply is preferably from 100 ms to 300 ms inclusive. In the above formulas, the last repetition of the electric current supply means the (N+1)th repetition of the electric current supply. If the period of the last repetition is less than 100 ms, the formation of the nugget is insufficient. If the period of the last repetition of the electric current supply exceeds 300 ms, the workability deteriorates, and the contribution of the electric current supply to the formation of the nugget is small. The period of the last repetition of the electric current supply may be selected optimally within the above range according to the period required for the first electric current supply and the subsequent repetitions of the electric current supply and the suspension of the electric current supply.

The resistance spot welding device according to embodiments of the present invention is not limited to the structures shown in FIGS. 4 and 5. It is only necessary for the resistance spot welding device according to embodiments of the present invention to include: the vertical pair of electrodes which holds parts to be welded and through which the electrode force and the electric current are applied; and the welding current controller that can control the welding current freely during welding. No particular limitation is imposed on the pressure mechanism (such as an air cylinder or a servo motor), the current control mechanism (such as an AC or DC current control mechanism), the type (a stationary type, a robot gun, etc.), etc.

In certain embodiments, the present invention is applied to a welding device for a sheet set of a plurality of sheets including a galvanized steel sheet or a high-strength steel sheet. Galvanized steel sheets and high-strength steel sheets are more likely to cause expulsion due to a sheet gap than ordinary steel sheets. However, since certain embodiments of the present invention has the effect of preventing the occurrence of expulsion. It is more effective to apply certain embodiments of invention to welding of a sheet set including at least one sheet selected from those steel sheets.

Therefore, even when at least one of the steel sheets included in the sheet set to be welded is a high-strength steel sheet having a tensile strength of 980 MPa or more, the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed.

Even when at least one of the steel sheets included in the sheet set to be welded is a high-strength steel sheet having a tensile strength of 980 MPa or more and containing components including 0.15≦C≦0.30 (% by mass), 1.9≦Mn≦5.0 (% by mass), and 0.2≦Si≦2.0 (% by mass), the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed.

Moreover, even when at least one of the steel sheets included in the sheet set to be welded is a galvanized steel sheet, the occurrence of expulsion is prevented, and a nugget with a large diameter can be formed. The galvanized steel sheet is a steel sheet including a coating layer containing Zn as a main component and encompasses any conventionally known galvanized layer. Specific examples of the coating layer containing Zn as a main component include a hot-dip galvanized layer, an electrogalvanized layer, an Al coating layer, a Zn—Al coating layer, and a Zn—Ni layer.

In the resistance spot welding device according to embodiments of the present invention, the electrode force is measured as described above, and the electric current is supplied and suspended while the electrode force is controlled appropriately according to the measured electrode force during the electric current supply. Thereby, the occurrence of expulsion is prevented, and a large nugget can be formed. Therefore, even when work disturbances such as a sheet gap are present, the diameter of the nugget can be ensured stably.

EXAMPLE 1

In Examples of the present invention, the device shown in FIG. 5 was used to produce a resistance spot-welded joint using a sheet set 3 including two overlapping hot-dip galvannealed steel sheets (a lower steel sheet 1 and an upper steel sheet 2). Specifically, the device used in the Examples was a welding device of the C gun type including a servo motor used to press the electrodes. The power source used was a DC power source.

In this case, the electric current was supplied under conditions shown in Table 1.

The electrodes 4 and 5 used were DR type electrodes made of alumina-dispersed copper and having a tip radius of curvature R of 40 mm and a tip diameter of 8 mm. The test pieces used were high-strength steel sheets having a 980 MPa-class tensile strength and sheet thicknesses of 0.8 to 2.6 mm and a high-strength steel sheet having a 1,470 MPa-class tensile strength and a sheet thickness of 2.0 mm. Two steel sheets of the same type and with the same thickness were stacked and welded.

The electrode force during the electric current supply was measured using the strain gauge 61 attached to the C gun. The electric current supplied was changed such that the measured electrode force was adjusted to a prescribed value.

Table 1 shows the results of studies on the occurrence of expulsion during welding and the diameter of the nugget. The diameter of the nugget was evaluated based on the structure of an etched cross section as follows. Let the sheet thickness be t (mm). Then a “Good” rating was given when the diameter of the nugget was equal to or larger than 5.5 √t. A “Poor” rating was given when the diameter of the nugget was less than 5.5 √t. Specifically, a nugget diameter equal to or larger than 5.5 √t was set to be an appropriate diameter.

TABLE 1 Test piece Initial First current Second current Evalu- Tensile Sheet electrode supply First suspension supply Occur- ation of strength thickness force Fi I₁ T₁ F_(h) ⁽¹⁾/ T_(c1) F_(c) ⁽¹⁾/ F_(c) ⁽¹⁾/ I₂ T₂ rence of nugget No. (MPa) (mm) (kN) (kA) (ms) F_(i) (ms) F_(i) F_(h) ⁽¹⁾ (kA) (ms) expulsion diameter Remarks 1 980 1.2 5.0 10 60 1.05 20 1.03 0.98 8.5 280 No Good Inventive Example 2 980 1.2 5.0 10 60 1.05 40 1.00 0.95 8.5 280 Yes Poor Comparative Example 3 980 1.2 5.0 10 60 1.01 20 1.00 0.99 8.5 280 No Poor Comparative Example 4 980 1.2 5.0 8.5 60 1.01 0 — — 8.5 220 Yes Poor Comparative Example 5 1470 2.0 6.0 9 60 1.07 20 1.04 0.98 7.8 300 No Good Inventive Example 6 1470 2.0 6.0 9 60 1.08 60 1.00 0.92 7.8 320 Yes Poor Comparative Example 7 1470 2.0 6.0 9 60 1.03 20 1.00 0.97 7.8 320 No Poor Comparative Example 8 1470 2.0 6.0 8 60 1.05 0 — — 7.8 260 Yes Poor Comparative Example 9 980 0.8 3.0 10 60 1.10 20 1.07 0.97 8.5 300 No Good Inventive Example 10 980 0.8 3.0 10 60 1.10 0 — — 8.5 300 Yes Poor Comparative Example 11 980 2.0 4.5 10 60 1.11 20 1.07 0.96 8.5 300 No Good Inventive Example 12 980 2.0 4.5 7.5 80 1.04 20 1.02 0.98 8.5 300 No Good Inventive Example 13 980 2.0 4.5 10 60 1.11 60 1.02 0.92 8.5 300 No Good Inventive Example 14 980 2.0 4.5 11.5 20 1.09 20 1.04 0.96 8.5 300 No Good Inventive Example 15 980 2.0 4.5 10 60 1.11 100 0.98 0.88 8.5 300 Yes Poor Comparative Example 16 980 2.0 4.5 8 120 1.16 20 1.13 0.98 8.5 300 No Poor Comparative Example 17 980 2.0 4.5 8.5 60 1.07 0 — — 8.5 300 Yes Poor Comparative Example 18 980 2.3 5.0 9.5 60 1.10 20 1.06 0.96 8 300 No Good Inventive Example 19 980 2.3 5.0 11 20 1.08 20 1.04 0.96 8 300 No Good Inventive Example 20 980 2.3 5.0 9.5 60 1.10 100 0.98 0.89 8 300 Yes Poor Comparative Example 21 980 2.3 5.0 7.5 120 1.08 20 1.07 0.99 8 300 Yes Poor Comparative Example 22 980 2.3 5.0 8 60 1.06 0 — — 8 260 Yes Poor Comparative Example 23 980 2.6 6.0 9.5 60 1.08 20 1.05 0.97 8 300 No Good Inventive Example 24 980 2.6 6.0 8 60 1.08 0 — — 8 260 Yes Poor Comparative Example

In Table 1, I₁ (kA) is the current value in the first electric current supply, T₁ (ms) is the current supply time in the first electric current supply, and F_(h) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(h) ⁽¹⁾ to the initial electrode force Fi. Tc₁ (ms) is the first suspension time, and F_(c) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(c) ⁽¹⁾ to the initial electrode force Fi. F_(c) ⁽¹⁾/F_(h) ⁽¹⁾ is the ratio of the electrode force ^(F) _(c) ⁽¹⁾ at which the electric current supply is resumed to the electrode force F_(h) ⁽¹⁾ at which the suspension of the electric current supply is started. I₂ (kA) is the current value in the second electric current supply, and T₂ (ms) is the current supply time in the second electric current supply.

As can be seen from Table 1, when the welding was performed using the resistance spot welding device according to embodiments of the present invention, no expulsion occurred, and each nugget formed had an appropriate diameter, in contrast to Comparative Examples.

EXAMPLE 2

In Examples of the present invention, a servo motor pressurizing-type resistance welding device attached to a C gun and including a DC power source was used to perform resistance spot welding on a sheet set including three overlapping hot-dip galvannealed steel sheets to thereby produce a resistance spot-welded joint.

In this case, the electric current was supplied under conditions shown in Table 2.

The electrodes 4 and 5 used were DR type electrodes made of alumina-dispersed copper and having a tip radius of curvature R of 40 mm and a tip diameter of 8 mm. The test pieces used were 980 MPa class high-strength steel sheets having sheet thicknesses of 0.8 to 2.3 mm and a 1,800 MPa class high-strength steel sheet having a sheet thickness of 1.2 mm. Three steel sheets of the same type and with the same thickness were stacked and welded.

The electrode force during the electric current supply was measured using a strain gauge attached to the C gun. The electrode force was changed such that the measured electrode force was adjusted to a prescribed value.

Table 2 shows the results of studies on the occurrence of expulsion during welding and the diameter of the nugget. The diameter of the nugget was evaluated based on the structure of an etched cross section as follows. Let the sheet thickness be t (mm). Then a “Good” rating was given when the diameter of the nugget was equal to or larger than 5.5 √t. A “Poor” rating was given when the diameter of the nugget was less than 5.5 √t. Specifically, a nugget diameter equal to or larger than 5.5 √t was set to be an appropriate diameter.

The same test was repeated 10 times, and the variations in nugget diameter were evaluated. When the diameters obtained were appropriate and the range of variations in nugget diameter was equal to or less than 0.1 √t, an “Excellent” rating was given.

TABLE 2 Test piece 1 Initial First current Second current Tensile Sheet electrode supply First suspension supply strength thickness force Fi I₁ T₁ F_(h) ⁽¹⁾/ T_(c1) F_(c) ⁽¹⁾/ F_(c) ⁽¹⁾/ I₂ T₂ No. (MPa) (mm) (kN) (kA) (ms) F_(i) (ms) F_(i) F_(h) ⁽¹⁾ (kA) (ms) 1 980 0.8 3.5 9.5 60 1.11 20 1.09 0.97 9   60 2 980 0.8 3.5 9.5 60 1.11 20 1.09 0.97 — — 3 980 0.8 3.5 9.5 60 1.11 20 1.09 0.97 6   120  4 980 0.8 3.5 8 60 1.08 0 — — — — 5 980 2.3 6.0 9 60 1.05 20 1.03 0.98 8.5 60 6 980 2.3 6.0 9 60 1.05 20 1.03 0.98 — — 7 980 2.3 6.0 9 60 1.05 20 1.03 0.98 6.5 120  8 980 2.3 6.0 9 60 1.05 0 — — — — 9 1800 1.2 5.5 9 60 1.07 20 1.05 0.98 8.5 60 10 1800 1.2 5.5 9 60 1.07 20 1.05 0.98 — — 11 1800 1.2 5.5 8 60 1.07 0 — — — — Third current Evalu- Second current Second suspension supply Occur- ation of supply T_(c2) F_(c) ⁽²⁾/ F_(c) ⁽²⁾/ I₃ T₃ rence of nugget No. F_(h) ⁽²⁾/F_(c) ⁽¹⁾ (ms) F_(c) ⁽¹⁾ F_(h) ⁽²⁾ (kA) (ms) expulsion diameter Remarks 1 1.08 20 1.05 0.98 8 240 No Excellent Inventive Example 2 — — — — 8 240 No Good Inventive Example 3 1.03 20 1.01 0.99 8 240 Yes Good Inventive Example 4 — — — — 8 240 Yes Poor Comparative Example 5 1.05 20 1.03 0.98 8 260 No Excellent Inventive Example 6 — — — — 8 260 No Good Inventive Example 7 1.03 20 1.02 0.98 8 260 No Good Inventive Example 8 — — — — 8 260 Yes Poor Comparative Example 9 1.05 20 1.03 0.98 8 280 No Excellent Inventive Example 10 — — — — 8 280 No Good Inventive Example 11 — — — — 8 280 Yes Poor Comparative Example

In Table 2, I₁ (kA) is the current value in the first electric current supply, T₁ (ms) is the current supply time in the first electric current supply, and F_(h) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(h) ⁽¹⁾ to the initial electrode force Fi. Tc₁ (ms) is the first suspension time, and F_(c) ⁽¹⁾/F_(i) is the ratio of the electrode force F_(C) ⁽¹⁾ to the initial electrode force Fi. F_(c) ⁽¹⁾/F_(h) ⁽¹⁾ is the ratio of the electrode force at which the electric current supply is resumed to the electrode force at which the suspension of the electric current supply is started. Similarly, F_(h) ⁽²⁾/F_(c) ⁽¹⁾ is the ratio of the electrode force at which, after the second electric current supply, the suspension of the electric current supply is started to the electrode force immediately after the first suspension. F_(c) ⁽²⁾/F_(c) ⁽¹⁾ is the ratio of the electrode force immediately after the second suspension to the electrode force immediately after the first suspension, and F_(c) ⁽²⁾/F_(h) ⁽²⁾ is the ratio of the electrode force immediately after the second suspension to the electrode force at which, after the second electric current supply, the suspension of the electric current supply is started. I₂ (kA) and I₃ (kA) are the current values in the second and third electric current supplies, respectively, T₂ (ms) and T₃ (ms) are the current supply times in the second and third electric current supplies, respectively, and Tc₂ (ms) is the second suspension time.

As can be seen from Table 2, when the resistance spot welding was performed according to embodiments of the present invention, no expulsion occurred, and each nugget formed had an appropriate diameter, in contrast to Comparative Examples. As can also be seen, when the second supply of the electric current was performed under the conditions of embodiments of the present invention, the effect of stabilizing the nugget diameter was obtained, in contrast to other cases.

REFERENCE SIGNS LIST

1 lower steel sheet

2 upper steel sheet

3 sheet set

4 lower electrode

5 upper electrode

6 electrode force gauge

61 strain gauge

7 controller

8 gun arm

9 nugget 

1. A resistance spot welding device for welding a sheet set including at least two overlapping steel sheets by holding the sheet set between a pair of welding electrodes and supplying an electric current to the sheet set under pressure, the resistance spot welding device comprising: a pair of electrodes; an electrode force gauge that measures an electrode force F applied by the electrodes; and a controller that controls an electric current supply to the electrodes according to the electrode force F measured by the electrode force gauge, wherein the controller controls the electric current supply such that, when the electrode force F measured by the electrode force gauge after the electric current supply is started changes from an initial electrode force Fi to an electrode force F_(h) ⁽¹⁾ represented by formula (1) 1.03×Fi≦F _(h) ⁽¹⁾≦1.15×Fi,   (1) while a lapse from the start of the electric current supply is between 20 ms and 80 ms inclusive, a suspension of the electric current supply of from 20 ms to 60 ms inclusive is started, and then, the electric current supply is resumed when the electrode force F reaches an electrode force F_(c) ⁽¹⁾ represented by formula (2): 1.01×Fi≦F _(c) ⁽¹⁾≦0.99×F _(h) ⁽¹⁾   (2).
 2. The resistance spot welding device according to claim 1, wherein the controller further controls the electric current supply such that, after the suspension of the electric current supply, the electric current supply of from 20 ms to 80 ms inclusive and the suspension of the electric current supply of from 20 ms to 60 ms inclusive are repeated at least once, and that, when the electrode force F during an Nth electric current supply changes from an electrode force F_(c) ^((N−1)) immediately after an (N−1)th suspension of the electric current supply to an electrode force F_(h) ^((N)) represented by formula (3), 1.04×F _(c) ^((N−1)) ≦F _(h) ^((N))≦1.15×F _(c) ^((N−1)),   (3) the suspension of the electric current supply is started, and then, the electric current supply is resumed when the electrode force F reaches an electrode force F_(c) ^((N)) represented by formula (4): F _(c) ^((N−1)) ≦F _(c) ^((N))≦0.99×F _(h) ^((N)),   (4) wherein N is a natural number of 2 or more.
 3. The resistance spot welding device according to claim 1, wherein the controller controls the electric current supply such that the period of a last (N+1)th repetition of the electric current supply is from 100 ms to 300 ms inclusive.
 4. The resistance spot welding device according to claim 1, further comprising a gun arm to which the pair of electrodes is mounted, wherein the electrode force gauge is a strain gauge that measures strain of the gun arm.
 5. The resistance spot welding device according to claim 2, wherein the controller controls the electric current supply such that the period of a last (N+1)th repetition of the electric current supply is from 100 ms to 300 ms inclusive.
 6. The resistance spot welding device according to claim 2, further comprising a gun arm to which the pair of electrodes is mounted, wherein the electrode force gauge is a strain gauge that measures strain of the gun arm.
 7. The resistance spot welding device according to claim 3, further comprising a gun arm to which the pair of electrodes is mounted, wherein the electrode force gauge is a strain gauge that measures strain of the gun arm.
 8. The resistance spot welding device according to claim 5, further comprising a gun arm to which the pair of electyrodes is mounted, wherein the electrode force gauge is a strain gauge that measures strain of the gun arm. 